Phase change film for semiconductor nonvolatile memory and sputtering target for forming phase change film

ABSTRACT

A phase change film for a semiconductor nonvolatile memory and a sputtering target for forming the phase change film. The phase change film for a semiconductor nonvolatile memory and the sputtering target for forming the phase change film have a composition containing 10 to 25 atomic % of Ge, 10 to 25 atomic % of Sb, 1 to 10 atomic % of Ga, and 10 atomic % or less of B, Al, C, Si and lanthanoid elements, with the balance being Te and inevitable impurities.

CROSS-REFERENCE TO PRIOR APPLICATION

This is a U.S. National Phase Application under 35 U.S.C. §371 of International Patent Application No. PCT/JP2004/013036 filed Sep. 8, 2004, and claims the benefit of Japanese Patent Application Nos. 2003-324063 filed Sep. 17, 2003 and 2004-102724 filed Mar. 31, 2004, both of which are incorporated by reference herein. The International Application was published in Japanese on March 31, 2005 as WO 2005/029585 al under PCT Article 21(2).

TECHNICAL FIELD

The present invention relates to a phase change film for a semiconductor nonvolatile memory and a sputtering target for forming the phase change film.

BACKGROUND ART

Phase change films for semiconductor nonvolatile memory (Phase Change RAM or PCRAM) has been used as recording layers. A phase change material in a crystalline state is used for the recording layers. In this case, rewrite is performed by rapidly heating and melting a portion of the phase change material with a heater, and then rapidly cooling the portion to make it partially amorphous, or otherwise slowly heating an amorphous portion at the temperature over its crystallization temperature and under its melting point, to bring it back to a crystalline state. Meanwhile, readout is performed due to difference between the electrical resistances of the phase change material in a crystalline state and a partially amorphous state. As one of the phase change films, there is known a phase change film having a composition containing 10 to 25% of Ge and 10 to 25% of Sb, with the balance being Te and inevitable impurities. There is also known a phase change film formed by performing sputtering using a target with almost the same component composition as the above phase-change recording layer. For example, see JP-W No. 2001-502848, JP-W No. 2002-512439, JP-W No. 2002-540605 “OYO BUTURI” (A monthly publication of The Japan Society of Applied Physics, Vol. 71, No. 12, 2002, p. 1513 to 1517

[Non-Patent Document 2] “Nikkei Micro-devices”, March issue in 2003, p.104

As disclosed in “OYO BUTURI” at the time of writing/erasing, it is first necessary to raise the temperature of crystal above a melting point, particularly, in order to change a crystalline state to an amorphous state (reset operation). In this case, if the melting point is high, the value of a current which is allowed to flow through a circuit should be large. As a consequence, the power consumption increases, and large current flow increases the load to a peripheral circuit, thereby to reduce the size of the circuit is prevented.

DETAILED DESCRIPTION OF THE INVENTION

Therefore, the inventors of the present invention have conducted research to solve such problems. As a result, the following research results were obtained.

When 1 to 10% of Ga is contained in an ordinary phase change film with a composition containing 10 to 25% of Ge and 10 to 25% of Sb, with the balance being Te and inevitable impurities, the melting point can be lowered and the electric resistivity of the film in a crystalline state barely changes. Therefore, the amount of the current needed for melting can be reduced. Thus, the power consumption can be reduced and a burden to a peripheral circuit can be relived.

Further, if the presence of B, Al, C, Si or a lanthanoid element is 10% or less of the film, the electric resistivity rises, and thus the amount of current needed for melting is further reduced. Accordingly, the power consumption can be reduced.

Among the lanthanoid elements, Dy, Tb, Nd, Sm, and Gd are particularly effective.

The present invention is achieved based on these research results, and is characterized by:

A phase change film for a semiconductor nonvolatile memory with a composition containing 10 to 25 atomic % of Ge, 10 to 25 atomic % of Sb, and 1 to 10 atomic % of Ga, with the balance being Te and inevitable impurities, and

A phase change film for a semiconductor nonvolatile memory with a composition 10 to 25 atomic % of Ge, 10 to 25 atomic % of Sb, 1 to 10 atomic % of Ga, and a total of 10 atomic % or less of at least one or more elements selected from a group consisting of B, Al, C, Si and lanthanoid elements, with the balance being Te and inevitable impurities.

Among the lanthanoid elements, Dy, Tb, Nd, Sm, and Gd are particularly preferable. Accordingly, the present invention is characterized by the phase change film for a semiconductor nonvolatile memory as described above in which the lanthanoid elements are at least one or more elements selected from a group consisting of Dy, Tb, Nd, Sm, and Gd.

In the phase change film for a semiconductor nonvolatile memory described above, preferably, the electric resistivity of the film measured by the four-point probe method after crystallization is 5×10⁻³ to 5×10 Ω·cm, and the melting point of the film is 600° C. or less. Accordingly, the present invention is characterized by a phase change film for a semiconductor nonvolatile memory described above, wherein the electric resistivity of the film measured by the four-point probe method after crystallization is 5×10⁻³ to 5×10 Ω·cm, and the melting point of the film is 600° C. or less.

The phase change film formed using the sputtering targets of the present invention enables a low melting point to be obtained without remarkably lowering resistance so much, and can reduce a current value at the time of writing operation, contribute to the reduction in power consumption and a miniaturization of devices, and make a great contribution to the development of a new semiconductor memory industry.

The reasons why the component compositions of the phase change film for a semiconductor nonvolatile memory according to the present invention are limited, as mentioned above, will be described.

When Ga component is contained in a phase change film with a composition containing 10 to 25% of Ge and 10 to 25% of Sb, with the balance being Te and inevitable impurities, Ga component has a function to further lower the melting point of the phase change film. However, if less than 1% of Ga is contained, the effect of lowering the melting point is little, which is not preferable. On the other hand, if Ga is contained over 10%, the crystallization temperature rises excessively, which is not preferable. A proper rise in the crystallization temperature improves the stability of an amorphous state which leads to improvement of the retention characteristics. However, if the crystallization temperature rises excessively, the electric power required for crystallization increases, which is not preferable from the viewpoint of a reducing power consumption. Accordingly, the amount of Ga to be contained in the phase change film is set to be 1 to 10% (more preferably, 2 to 8%).

In addition, even if 1 to 10% of Ga is contained in the phase change film of a semiconductor nonvolatile memory with a composition containing 10 to 25% of Ge and 10 to 25% of Sb, with the balance being Te and inevitable impurities, the electric resistivity of the film in its crystallized state is not lowered.

Further, the phase change film with a composition containing 10 to 25% of Ge and 10 to 25% of Sb, with the balance being Te and inevitable impurities, has mainly two types of crystal structures, i.e., a face-centered cubic crystal structure having a high resistance and a hexagonal crystal structure having a low resistance. The face-centered cubic crystal structure is created when the film is crystallized at a relatively low temperature, and the hexagonal crystal structure is created when the film is kept at a relatively high temperature. Since the phase change rate from an amorphous state to a face-centered cubic crystal state is rapid, the crystal which is created when the film is phase-changed and crystallized from an amorphous state is generally face-centered cubic crystal. However, if Ga is added to the conventionally known composition of Ge—Sb—Te, the face-centered cubic crystal structure is stabilized up to a high temperature as compared with the case of not adding Ga. Therefore, Ga also has an effect of improving the temperature stability of the electric resistivity.

Because B, Al, C, Si, and lanthanoid elements have a function to further raise a resistance value in a crystalline state of the phase change film by the addition of Ga, they are added, if necessary. However, if these components are contained over 10%, the rise in the crystallization temperature of the phase change film increases excessively, which is not preferable. A proper rise in the crystallization temperature improves the stability of an amorphous state which leads to improvement of the retention characteristics. However, if the crystallization temperature rises excessively, the electric power required for crystallization increases, which is not preferable from the viewpoint of reducing power consumption. Accordingly, the content of these components are set to be 10% or less. The range of the content is more preferably 0.5 to 8%. In addition, among the lanthanoid elements, Dy, Tb, Nd, Sm, and Gd are particularly preferable.

Ge and Sb contained in the phase change film having a high electrical resistance according to the present invention is preferably 10 to 25% of Ge and 10 to 25% of Sb. The reason is based on the fact that, if Ge is less than 10% and Sb is less than 10% and if Ge is over 25% and Sb is over 25%, the resistance value becomes low and the crystallization time becomes long, which are not preferable.

The phase change film according to the present invention requires the electric resistivity value measured by the four-point probe method after crystallization to be 5×10⁻³ Ω·cm or more (more preferably, 8×10⁻² Ω·cm or more). The reason comes from the fact that, if the electric resistivity value is less than 5×10⁻³ Ω·cm, a large current flows through a circuit, which therefore increases the power consumption and becomes an obstacle in reducing the size of the circuit, which are not preferable. Further, the electric resistivity of a Ge—Sb—Te alloy in an amorphous state is generally about 1×10² Ω·cm. It is preferable that this alloy has a difference of about at least one and a half digits between the resistivities of the alloy in a crystalline state and an amorphous state for stable read-out. Therefore, the resistivity value of the phase change film in a crystalline state is required to be 5×10 Ω·cm or less. Accordingly, the electric resistivity measured by the four-point probe method after the crystallization of the phase change film according to the present invention is set to be 5×10⁻³ Ω·cm to 5×10 Ω·cm. Moreover, the melting point of the phase change film according to the present invention is required to be 600° C. from the viewpoint of low power consumption.

A sputtering target for forming a phase change film for a semiconductor nonvolatile memory with the composition, as described above according to the present invention, can have a component composition containing 10 to 26 atomic % of Ge, 10 to 26 atomic % of Sb, and 1 to 11 atomic % of Ga, with the balance being Te and inevitable impurities.

Further, a sputtering target for forming a phase change film for a semiconductor nonvolatile memory with the composition, as described above can have a composition 10 to 26 atomic % of Ge, 10 to 26 atomic % of Sb, 1 to 11 atomic % of Ga, and a total of 11 atomic % or less of at least one or more elements selected from a group consisting of B, Al, C, Si and lanthanoid elements, with the balance being Te and inevitable impurities.

Accordingly, the present invention includes includes a sputtering target for forming a phase change film for a semiconductor nonvolatile memory with a composition containing 10 to 26 atomic % of Ge, 10 to 26 atomic % of Sb, and 1 to 11 atomic % of Ga, with the balance being Te and inevitable impurities.

Another sputtering target for forming a phase change film for a semiconductor nonvolatile memory can include a composition 10 to 26 atomic % of Ge, 10 to 26 atomic % of Sb, 1 to 11 atomic % of Ga, and a total of 11 atomic % or less of at least one or more elements selected from a group consisting of B, Al, C, Si and lanthanoid elements, with the balance being Te and inevitable impurities, and a further sputtering target for forming a phase change film for a semiconductor nonvolatile memory as described above in which the lanthanoid elements are at least one or more elements selected from a group consisting of Dy, Tb, Nd, Sm, and Gd.

The sputtering target for forming a phase change film for a semiconductor nonvolatile memory having the component composition, as described above according to the present invention, is manufactured by melting a Ge—Sb—Te based alloy with a predetermined component composition in an Ar gas atmosphere, then adding Ga to the molten metal, pouring the molten metal into molds made of iron to manufacture an alloy ingot, pulverizing the alloy ingot in an inert gas atmosphere to manufacture an alloy powder having a particle size of 200 μm or less, and finally hot pressing the alloy powder in a vacuum. The vacuum hot pressing is performed by keeping the alloy powder under the following conditions: a pressure of 146 to 155 MPa, a temperature of 370 to 430° C., and a duration of 1 to 2 hours, and thereafter cooling the molds to a normal temperature at a cooling rate of 1 to 3° C./min when the temperature of the molds has dropped to 270 to 300° C.

Moreover, the sputtering target for forming a phase change film for a semiconductor nonvolatile memory having the component composition, as described above according to the present invention, is manufactured by adding Ga to a Ge—Sb—Te based alloy, mixing this alloy powder with one or more of the separately manufactured powders of B, Al, C, Si, and lanthanoid elements (preferably, Dy, Tb, Nd, Sm, and Gd) each having a particle size of 200 μm or less so as to have component compositions according to the present invention, and hot-pressing the alloy powder in a vacuum. The vacuum hot pressing is performed by keeping the alloy powder under the following conditions: a pressure of 146 to 155 MPa, a temperature of 370 to 430° C., and a duration of 1 to 2 hours, and thereafter cooling the molds to a normal temperature at a cooling rate of 1 to 3° C./min when the temperature of the molds has dropped to 270 to 300° C.

EXAMPLES OF THE INVENTION

Ge, Sb, and Te were melted in an Ar gas atmosphere. Ga was added to the obtained molten metal. An alloy ingot was manufactured by casting the molten metal obtained by adding Ga. An alloy powder having a particle size of 100 μm or less was manufactured by reducing the alloy ingot to powder in an Ar atmosphere. Mixed powders were manufactured by mixing the alloy powder with the respective elemental powders of B, Al, C, Si, Dy, Tb, Nd, Sm, and Gd.

Hot pressed bodies were manufactured by hot-pressing the alloy power and the respective mixed powders in a vacuum at a temperature of 400° C. and at a pressure of 146 MPa. Targets 1 to 21 according to the present invention, comparative targets 1 to 10, and conventional target 1 having the following dimensions: a diameter of 125 mm and a thickness of 5 mm, a disk shape, and component compositions as shown in Table 1 were manufactured by performing grinding processing on the hot pressed bodies under the condition of a lathe revolution speed of 200 rpm, using a carbide turning tool. TABLE 1 Component Composition (Atomic %) B, Al, C, Si, Target Ge Sb Ga Lanthanoid Element Te Present 1 22 22   1.2 — Balance Invention 2 21.9 21.9   1.5 — Balance 3 21.8 21.8 2 — Balance 4 21.5 21.5 3 — Balance 5 21.1 21.1 5 — Balance 6 20.4 20.4 8 — Balance 7 20 20 10  — Balance 8 20.8 20.8 4 B: 2.5 Balance 9 20.2 20.2 5 Al: 3.8 Balance 10 19.8 19.8 8 Si: 3.0 Balance 11 20.9 20.9 4 C: 2.0 Balance 12 18.8 18.8 10  Dy: 5.5 Balance 13 19.7 19.7 7 Tb: 1.5 Balance 14 19.8 19.8 3 Nd: 8.0 Balance 15 20.3 20.3 5 Sm: 3.5 Balance Present 16 20 20 6 Gd: 4.0 Balance Invention 17 20.9 20.9 3 B: 0.5, Al: 1.0 Balance 18 20.0 20.0 4 Al: 5, C: 1 Balance 19 20.2 20.2 5 C: 1, Si: 5, Dy: 2 Balance 20 19.8 19.8 2 Sm: 2, Tb: 5, Al: 2 Balance 21 19.0 19.0 7 B: 2, Si: 2, Balance Dy: 0.5, Sm: 3, Balance Comparative 1 22.1 21.1   0.3* — Balance 2 19.1 19.1 14* — Balance 3 17.3 17.3 9 B: 13* Balance 4 17.5 17.5 8 Si: 13* Balance 5 18.8 18.8 3 Gd: 12* Balance Comparative 6 17.8 17.8 5 C: 15* Balance 7 18.0 18.0 7 Al: 12* Balance 8 18.2 18.2 4 Dy: 14* Balance 9 18.7 18.7 2 Nd: 14* Balance 10 17.8 17.8 7 Tb: 13* Balance Conventional 22.2 22.2 — — Balance Target 1 Asterisk (*) means a value out of the range of the present invention.

Next, each of the targets 1 to 21 according to the present invention, comparative targets 1 to 10, and conventional target 1 is bonded to a cooling backing plate made of copper, with an indium solder material having a purity of 99.999% by weight. Then, the resulting targets are loaded into a direct-current magnetron sputtering apparatus within which the distance between the targets and substrates (Si wafers on the surface of each of which an SiO₂ film having a thickness of 100 nm is formed) is set to be 70 mm. Thereafter, the sputtering apparatus is vacuumed until the degree of an ultimate vacuum thereof becomes 5×10⁻⁵ Pa or less. Thereafter, the sputtering apparatus is supplied with Ar gas until the total pressure thereof become 1.0 Pa.

Substrate temperature: room temperature

Input power: 50 W (0.4 W/cm²)

Then, sputtering was performed under the above conditions, thereby forming phase change films 1 to 21, comparative phase change films 1 to 10, and conventional phase change film 1, which have a thickness of 300 nm and have component compositions as shown Tables 4 to 6 on the surfaces of the substrates.

The component compositions of the phase change films 1 to 21, comparative phase change films 1 to 10, and conventional phase change film 1, which were obtained in this way, were measured by an inductively coupled plasma (ICP) method. The results thereof are shown in Table 2.2 Moreover, the phase change films 1 to 21 according to the present invention, conventional phase change films, and conventional phase change film 1, which were obtained, were kept and crystallized in a nitrogen flow at a temperature of 230° C. for five minutes. Thereafter, electric resistivities were measured by a four-point probe method. Further, a film having a thickness of 3 μm was formed on a polycarbonate substrate having a diameter of 120 mm under the conditions described above. All of the formed film was peeled off and powderized. Then, the crystallization temperatures and melting points of the powdered materials were measured under the following conditions: an Ar flow rate of 200 ml/min and a rising temperature of 10° C./min, by a differential thermal analysis (DTA) method. The results thereof are shown in Table 2.2 In addition, the masses of samples used in this measurement are standardized as 15 mg. It should be noted herein that an exothermic peak appearing in the vicinity of 160 to 340° C. is used as the crystallization temperature and an endothermic peak appearing in the vicinity of 540 to 620° C. is used as the melting point. TABLE 2 Component Composition (Atomic %) Specific Crystallization Melting Phase B, Al, C, Si, Lanthanoid Resistance × 10⁻² Temperature Point Change Film Ge Sb Ga Element Te (Ω · cm) (° C.) (° C.) Present Invention 1 21.8 22.1 1.1 — Balance 1.89 170.3 595.8 2 21.3 22.0 1.3 — Balance 1.87 174.0 593.1 3 21.2 21.7 1.8 — Balance 1.94 178.7 586.9 4 21.6 21.2 2.4 — Balance 1.96 188.1 581.3 5 20.3 21.0 4.5 — Balance 2.03 206.8 570.8 6 20.2 20.9 7.1 — Balance 1.91 234.8 555.4 7 19.6 20.1 9.0 — Balance 1.87 253.5 547.2 8 20.8 20.3 3.2 B: 2.1 Balance 7.98 199.4 575.2 9 19.9 20 4.6 Al: 3.0 Balance 10.3 210.1 565.1 10 19.8 20.1 7.9 Si: 2.5 Balance 9.95 246.8 544.7 11 20.5 20.2 3.5 C: 0.9 Balance 11.1 211.5 576.1 12 18.1 19 8.7 Dy: 4.7 Balance 31.3 286.2 536.3 13 19.9 20.2 6.8 Tb: 0.6 Balance 26.8 231.2 554.2 14 19.2 19.5 2.9 Nd: 7.1 Balance 57.9 202.7 574.1 15 20.4 20.1 4.3 Sm: 2.9 Balance 22 213.1 568.8 Present Invention 16 19.1 19.8 5.2 Gd: 3.1 Balance 25.6 236.6 562.4 17 21.3 20.8 3.1 B: 0.3, Al: 0.5 Balance 18.4 228.6 573.4 18 19.8 20.3 3.8 Al: 4.0, C: 0.7 Balance 20.7 241.2 562.4 19 20.4 20.2 4.7 C: 0.5, Si: 1.2, Balance 23.4 224.1 566.4 Dy: 0.6 20 19.9 20.1 1.8 Sm: 1.4, Tb: 4.1, Balance 31.9 255.5 584.3 Al: 1.7 21 19.1 18.6 6.2 B: 1.5, Si: 1.4, Balance 38.4 268.9 554.7 Dy: 0.4, Sm: 2.1 Comparative 1 22 21.5 0.2* — Balance 1.88 167.5 606.5 2 18.3 19.5 12.8* — Balance 1.97 337.4 538.2 3 17.5 17.3 8.6 B: 12.4* Balance 52.8 301.4 546.9 4 17.1 17.6 7.8 Si: 12.0* Balance 36.8 314.7 545.6 5 17.5 17.5 2.7 G: 11.8* Balance 94.3 315.4 578.3 6 17.5 18.1 4.7 C: 13.8* Balance 84.5 325.6 564.1 7 17.3 18.1 6.8 Al: 11.3* Balance 54.8 314.3 549.3 8 18.4 18.3 3.3 Dy: 13.9* Balance 103.5 309.1 572.1 9 18.4 19.0 1.3 Nd: 13.1* Balance 124.7 312.4 586.4 10 17.5 17.1 6.5 Tb: 12.3* Balance 115.8 321.3 551.7 Conventional 22.3 21.7 — — Balance 1.74 162.3 613.4 Target 1 Asterisk (*) means a value out of the range of the present invention.

It can be understood from the results shown in Table 2 that the crystallized phase change films 1 to 21 according to the present invention, which were obtained by performing sputtering using the targets 1 to 21 according to the present invention, are excellent phase change films having lower melting points and having little drop in electric resistivities, as compared with the conventional phase change film 1, which was obtained by performing sputtering using the conventional target 1. However, it can be understood that at least one unfavorable characteristic appears in the comparative phase change films 1 to 10 containing additive components out of the range of this invention. 

1. A phase change film for a semiconductor nonvolatile memory having a composition comprising: 10 to 25 atomic % of Ge; 10 to 25 atomic % of Sb; 1 to 10 atomic % of Ga; and a balance of Te and impurities.
 2. A phase change film for a semiconductor nonvolatile memory comprising: 10 to 25 atomic % of Ge; 10 to 25 atomic % of Sb; 1 to 10 atomic % of Ga; a total of 10 atomic % or less of at least one or more elements selected from a group consisting of B, Al, C, Si and lanthanoid elements; and a balance of Te and impurities.
 3. The phase change film for a semiconductor nonvolatile memory according to claim 2, wherein the lanthanoid elements are at least one or more elements selected from a group consisting of Dy, Tb, Nd, Sm, and Gd.
 4. The phase change film for a semiconductor nonvolatile memory according to claim 1, wherein the electric resistivity of the film measured by a four-point probe method after crystallization is 5×10⁻³ to 5×10 Ω·cm, and the melting point of the film is 600° C. or less.
 5. A sputtering target for forming a phase change film for a semiconductor nonvolatile memory having a composition according to claim
 1. 6. A sputtering target for forming a phase change film for a semiconductor nonvolatile memory having a composition according to claim
 2. 7. The sputtering target for forming a phase change film for a semiconductor nonvolatile memory according to claim 6, wherein the lanthanoid elements are at least one or more elements selected from a group consisting of Dy, Tb, Nd, Sm, and Gd.
 8. The phase change film for a semiconductor nonvolatile memory according to claim 2 wherein the electric resistivity of the film measured by a four-point probe method after crystallization is 5×10⁻³ to 5×10 Ω·cm, and the melting point of the film is 600° C. or less. 